7. Tourmaline
Tourmalines are minerals of boron silicate group, and are used as gemstones because of their clarity and very wide range of colors. Tourmaline minerals are found as accessory minerals in some rocks (mostly granite, pegmatite and gneiss), and can be found in alluvial deposits because they are very durable and resistant, but the most beautiful are those that form as individual crystals due to the hydrothermal processes. Because of a great variety of tourmaline (since they differ chemically, but have same crystalline structure) there are many varieties: rubelite, schorl, indicolite, Paraiba and many more (those are tourmaline gemological names and not real mineralogical terms).
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